The present invention relates to a method and an installation for separating a mixture of hydrogen and carbon monoxide. In particular, it relates to a method for separating such a mixture using a step of separation by cryogenic distillation.
Carbon monoxide and hydrogen production units can be divided into two parts:                Generation of synthesis gas (mixture primarily containing H2, CO, CH4, CO2 and N2). Among the various industrial methods for producing synthesis gas, steam reforming is the most important. The design of this unit, which comprises a furnace, is based on the required production of CO and hydrogen.        Production of synthesis gas. This comprises:        an amine scrubbing unit to remove most of the CO2 present in the synthesis gas;        a unit for purification on adsorbent bed. This unit generally comprises two bottles in continuous operation, one in production, the second in regeneration phase;        a unit for low temperature treatment by a cryogenic method (cold box) in order to produce carbon monoxide and hydrogen (possibly including a mixture of carbon monoxide and hydrogen called Oxogaz) in the quantities and purities required by the consumer. The most common method is scrubbing with liquid methane to obtain pure carbon monoxide with a recovery yield up to 99%, hydrogen the CO content of which generally varies between a few ppm and 1%, and a methane rich waste gas used as fuel.        
Methods of this type are described in “Tieftemperaturtechnik” by Hausen et al., Springer-Verlag 1985 pp 417-419, EP-A-837031, EP-A-0359629, EP-A-0790212 and EP-A-1245533.
The thermodynamic equilibrium of the synthesis gas generation unit is favored by low pressure, which results in a lower consumption of raw material, while the synthesis gas purification unit is favored by high pressure in terms of equipment size and electric power consumption.
This is why, and due to the limitation of the operating pressure of reforming furnaces (which operate at a pressure below 45 bar abs.), it may be advantageous and/or necessary to incorporate a synthesis gas compressor in the synthesis gas purification line.
In most cases, the hydrogen produced by the cold box, containing up to 1 mol % of CO, is used as regeneration gas for the purification, and is then sent to an adsorbent purification unit (PSA) before being sent to the end consumer.
In the case in which the hydrogen produced by the cold box is sent directly to the consumer with a CO content specification of a few ppm, this gas can no longer be used as regeneration gas.
Also in the case in which a mixture of carbon monoxide and hydrogen is produced, generally containing 50% hydrogen, the quantity of hydrogen remaining as waste gas is too small to regenerate the purification; it is therefore necessary to find another gas as a regeneration gas.